Currently, the method of structuring the electrodes of a capacitive touch screen consists in depositing on a transparent substrate, for example made of glass, a layer of transparent conductive oxide such as indium-tin-oxide or ITO. After deposition, the ITO layer is removed from the entire surface of the subjacent substrate except at the places where electrodes have to remain. The substrate thus has large surfaces free of ITO, which must be compensated for optically since the refractive indices of the substrate and ITO are not identical. This optical compensation is performed by the deposition of thin non-conductive layers whose refractive indices are adapted to those of the substrate and ITO. Despite all the precautions taken, these optically compensated zones remain visible from certain angles of sight or in certain lighting conditions.
In order to overcome this problem, it has been proposed to remove the ITO only from along the contour of the electrodes. This solution is known, for example, from JP 60-260392 and WO 92/13328. The electrodes can be trimmed by photolithography or by laser ablation. The width of the trimming line can be made sufficiently thin (around 20 μm) that optical compensation for said line via depositions of dielectric layers with suitable optical indices does not raise any particular problem. One could even envisage making the trimming line so thin (width of the order of 5 μm) that the latter is invisible to the naked eye, without the necessity of providing additional optical compensation layers.
However, the thinner the trimming line, the greater the risk that small fragments of conductive material, capable of causing short-circuits, will remain. In order to overcome this problem, JP 60-260392 proposes cleaning the substrate, after laser ablation, by means of ultrasounds in a bath of cleaning fluid, such as alcohol. However, wet phase treatments are usually avoided since they slow down the rate of output and thus increase production costs.
Another drawback of the aforementioned method lies in the fact that stray capacitive couplings can occur between the electrodes which are brought to a fixed potential and the rest of the ITO surface, which is floating. Such stray capacitive couplings are capable of disturbing the proper operation of the capacitive touch screen, especially when the signal addressing frequency of the electrodes is high.
In order to overcome the aforementioned drawbacks, in addition to others, the present invention proposes a manufacturing method that enables the wet phase cleaning to be omitted.